The present invention relates to the stabilization of ink concentration in a solid ink add system and, more particularly, but not exclusively to the stabilization of ink concentration in the ink reservoir of a printer, wherein the reservoir is fed from a can of ink powder or from a suspension of ink powder.
The ink feed system in which solid ink is added to a reservoir of ink carrier or solvent is known as the solid add system. Using the solid add system, the idea is to control the viscosity in the reservoir to ensure stable color. In general printers are provided with a pre-designed ink feed cycle. During the cycle a solid add valve which is located between the ink can and the reservoir is open for a certain amount of time. Air pressure within the printer ensures that powder is fed from the can into the reservoir when the valve is open.
The extent to which the valve is open, the duty cycle of the valve, is a design feature of the particular printer. In designing the duty cycle an assumption is made that the current printing operation requires high levels of ink, that is high coverage. That is to say the duty cycle is chosen based on a worst case scenario as regards ink demand. The result is that in many cases the solid concentration in the ink becomes too high. Other assumptions are made regarding air pressure, amount of solid in the can etc.
In the typical solid ink add system, the amount of solid that is added over the duty cycle can vary strongly, depending on such diverse factors as the system air pressure, the coverage intensity required by the current print operation and paste viscosity in the can. Paste viscosity depends on the solid ink can state, that is whether it is full or empty, and even on whether it was shaken prior to installation. Viscosity in the reservoir depends on the amount of solid added but also on the sum total of Non Volatile Solid (NVS) concentration in the reservoir. The system is designated to support high coverage separations, that is printings of up to 100% coverage. Thus, in cases of low coverage and/or unexpected settings of the other parameters referred to above, there is an overshoot in the solid concentration in the tank.
While existing printers are able to measure the viscosity in the reservoir and apply or not apply ink feed, the duty cycle itself is fixed, often leading to instability in the viscosity within the reservoir.
Some printers have the capability to notify a user when the amount of powder in the ink can is too small to achieve the design viscosity, so that the user can change the can.